Most forebrain interneurons originate in the developing medial ganglionic eminence (MGE), from where they migrate into cortex, hippocampus, striatum, and amygdala to form local inhibitory circuits. When transplanted into the juvenile or adult mouse cortex, MGE cells retain the ability for migration, functional integration, and differentiation primarily into parvalbumin (PV) and somatostatin (SOM) expressing GABAergic cells. Previous work has shown that GABAergic inhibition is required for the induction of cortical plasticity and brain repair. Recent work from our laboratories has shown that transplantation of MGE cells into the neonatal or juvenile mouse visual cortex can induce a new period of ocular dominance plasticity (ODP). The transplantation of MGE cells can also enhance the brain's capacity for functional recovery and is being developed as a possible cell therapy for several brain disorders. The ability of these cells to induce plasticity de novo also offers a powerful tool to study the mechanisms and limits of cortical plasticity. The present proposal has three Aims. Aim (1) will determine which type of cortical interneuron is responsible for the induction of cortical plasticity. We have developed and validated genetic tools to ablate PV, SOM, or both cell types from the MGE grafts. Previous research suggests that PV cells may be responsible for the induction of ODP, but this hypothesis has not been formally tested. Our preliminary studies suggest that ODP can still be rekindled, even when most PV cells are eliminated from MGE grafts. We will determine if SOM interneuron depletion is sufficient for the elimination of ODP, or whether both these populations have the capacity to induce ODP. This ODP is induced in young animals weeks after the end of the critical period, but it remains unknown if the adult brain is similarly receptive to interneuron-induced plasticity. Aim (2) will determine if the transplantation of cortical interneurons in the adult brain can induce ODP and contribute to recovery of function. We have developed and validated optical recording techniques to study ODP induction in adult mice. We also have preliminary evidence that MGE cells grafted into the adult mouse cortex migrate and integrate, suggesting that they could also modify cortical circuits and possibly induce ODP. Aim (3) will determine the normal pattern of interneuron maturation in the human visual cortex. If cortical interneurons are key to the induction of critical period plasticity in humans, what is normal pattern of interneuron maturation in infants? Studies from our labs suggest that interneurons continue to be recruited into some regions in the infant brain and this could underlie extended periods of plasticity. Using a collection of brain samples from our developmental tissue bank, we have validated staining and stereological methods to quantify and map PV, SOM, and other markers of immature and mature interneurons. Identification of cortical interneurons responsible for the induction of plasticity, the age range when plasticity can be induced, and the normal patterns of human interneuron maturation will provide new information for the use of MGE cells in brain repair.